One method for converting an analog signal to a digital representation is the SinCT method. The SinCT method compares the analog signal to a reference signal, and uses the time stamp of the time when the magnitude relationship of the two signals satisfies the prescribed condition to convert an analog signal to a digital amplitude sequence (i.e., see Unexamined Japanese Patent Publication (Kokai) No. 2003-124,810 or U.S. Pat. No. 6,717,540).
The information obtained by the SinCT method includes the time stamps having an irregulary interval and the amplitudes of the reference signal corresponding to these time stamps. In other words, the SinCT method differs from a conventional analog-to-digital conversion method; the sampling time interval is not constant. On the other hand, generally, the result of the analog-to-digital conversion often requires the amplitudes at times having a constant interval. For example, the fast Fourier transform (FFT) requires a collection of the digital amplitudes at times having a constant interval. Consequently, a process is needed to convert the information obtained by the SinCT method into amplitudes at times having a constant interval. This conversion process is referred to as gridding. For example, gridding is performed by interpolation or polynomial approximation, and needs a long computation time or many computational resources. For example, a 1024-point gridding requires several seconds of processing time when processing on a computer equipped with a processor having a clock frequency of 1 GHz. When digital automatic test equipment (digital ATE) is used to obtain a digital amplitude sequence at times having a constant interval from an analog signal, a shorter time required for gridding is desired.